Android P is coming to the OnePlus 3 and 3T soon, confirms a post on the OnePlus Forum by Gary C., who is the operations manager of Oxygen OS. Gary confirms that Android P will come in OnePlus 6, and OnePlus 5/5T and then to the OnePlus 3/3T in order, when stable.
The OnePlus 6 and OnePlus 5/5T currently runs on Android 8.1 Oreo, while the OnePlus 3/3T on Android 8.0 Oreo. So, OnePlus is directly jumping to Android P (i.e. 9.0) upgrade for its 3 and 3T, skipping Android 8.1 version. Also, the OnePlus 6 has already an access to the beta version of Android P and thus will be the first device of the company to get a stable version of it.
However, there are no clear indications of the OnePlus 2, OnePlus X, and OnePlus One, getting an Android P upgrade.
On Android P upgrade to the OnePlus 3 and 3T, Gary stated, “We have decided to focus our development resources on the Android P project instead of upgrading the device to Android 8.1 so that we can offer more exciting features and improvements for our OnePlus 3 and 3T users.”
Again, with no indications of the launch of a stable version of Android P anytime soon, we may expect it to be featured in OnePlus devices towards the end of 2018 or even in early 2019.